docs: add CONTRIBUTORS.md issues-only policy (closes #1517)

[cliffhall]

Closes #1517

Adds CONTRIBUTORS.md (issues-only policy + version→board/branch/label table), the PR-template pointer, and an AGENTS.md ## Contributing link.

Provenance: this work was briefly folded into the Wave-1 rollup (1579-wave-1 / #1601) during review, then pulled back out — it's unrelated to the #1510 Apps-host decomposition that defines Wave 1. This PR is the real, standalone path to v2/main. Clean history (cherry-picked the doc commits onto v2/main, no rollup/merge/empty-commit noise). The old #1537 shows "merged into 1579-wave-1" from that interim step and is superseded by this.

[BobDickinson]

I like this. I might remove the v1.5 references (and maybe kill the v1.5 branch altogether), since nobody outside of the core team probably ever saw that of knows what it is/was.

It might also be good to say that if you want to work on the project as part of the team, that we invite you to join us on Discord or attend our meetings. It seems like we need some way for someone who really is interested in volunteering either in general or in some specific area to be able to contribute under our supervision, with a possible path to becoming a maintainer. I'm not sure what that looks like, but I'm trying to make it seem like we've completely closed the door to people we really want to contribute a level that makes sense for us to support.

[cliffhall]

Both addressed in the latest commit.

1. v1.5 references removed. You're right that it's internal history nobody outside the core team ever saw — surfacing it in an outward-facing doc is just noise. The version table is now v1 / v2 only, and the "Label by version" line drops v1.5 too. I left the v1.5 notes in AGENTS.md alone: that's the maintainer-facing doc, and the "frozen, kept as a reference point, do not delete" note there is still useful to us (and to an agent) when chasing a regression from the merge into v2/main. Killing the branch itself is a separate call — happy to open an issue for it if you want to go that way.

2. Door left open. Added a "Want to work on the Inspector with us?" section that frames the issues-only rule as being about unsolicited patches, not about turning people away:

• Join the MCP Contributor Discord and say hello in #inspector-dev — the existing channel for day-to-day Inspector development.
• Attend the community calls at meet.modelcontextprotocol.io, with a pointer to the org's Contributor Communication page for the full channel list.
• Explicit framing: from there we can scope a piece of work with you and supervise it through our workflow, "which is also the path toward becoming a maintainer" — contrasted against drive-by diffs. Plus a Code of Conduct link.

It's linked from "Why this policy exists" ("we'd rather bring you into the workflow than turn you away"), and I added the same #inspector-dev pointer to the PR template so someone who gets bounced from opening a PR immediately sees where to go instead.

Existing channels are used rather than inventing new ones, so nothing here needs standing up. If you'd rather point at a dedicated Inspector meeting slot or a different escalation path, tell me what it is and I'll swap it in.